The present invention relates generally to acoustic systems and methods for non-destructive evaluation of materials, and more particularly to scanning acoustic microscopy of materials utilizing a differential property sensitive acoustic lens.
A scanning acoustic microscope (SAM) is used effectively for evaluating elastic properties of materials in the microscopic non-destructive inspection of materials. The conventional SAM includes an acoustic lens consisting essentially of a single spherical concave surface ground at one flat end of a cylindrical rod and a piezoelectric transducer attached to the other flat end of the rod. The concavity of the lens is filled with or immersed in a coupling fluid contacting a sample of the material under evaluation. A radio frequency signal excites the transducer and produces acoustic waves which propagate along the rod and converge to a diffraction limited spot on or just below the surface of the sample. The coupling fluid (usually water, oil, gel, alcohol, methanol, mercury, liquid helium or a solid coupler) transmits the acoustic waves from the rod at the lens into the sample. A signal reflected by the surface or an anomaly inside the sample is transmitted back through the coupling fluid and is propagated back to the transducer. A single transducer may be used to transmit and receive signals. The sample is scanned using mechanical scanning means, and the signals are electronically processed and an acoustic image is constructed point by point in a raster pattern which is recorded or displayed on a monitor.
Contrasts in the observed acoustic images can be related to local differences in elastic properties of the material. Contrast enhancement is achieved by modifying the acoustic lens to include angles large enough to generate surface acoustic waves (SAW) on the sample. The SAWs are scattered by surface inhomogeneities and contribute extra signals to the transducer and thereby enhance image contrast. Large opening angle (30 to 60xc2x0) lenses are routinely used in the SAM procedure for surface defect characterization. Although SAWs enhance the detectability of surface and near surface defects, there is a substantial reduction in the subsurface defect detection capability of the SAM procedure, because the SAWs carry away a large portion of the incident acoustic energy and leave only a small amount of energy to propagate into the sample for internal flaw detection. Acoustic transducers with small opening angles (5 to 20xc2x0) are therefore typically used in subsurface imaging applications, but surface and subsurface images with large contrast are difficult to obtain using small opening angles. Contrast enhancement may also be achieved by performing differential amplitude and differential phase imaging on the sample. Usually, in conventional acoustic microscopes as well as in C-Scan, only the amplitude of the reflected signal is used, and the phase information is often not detected and is usually discarded. An important difficulty with acoustic differential amplitude or acoustic phase imaging is the inability to extract a reference signal from a spot close to the region of interest on the sample.
The invention solves or substantially reduces in critical importance problems with prior art SAM systems and procedures as just described by providing a differential property sensitive acoustic lens for use in nondestructive materials evaluation. A SAM incorporating the lens of the invention explicitly permits measurement of acoustic signal phase information as well as signal amplitude in an acoustic image, which results in enhanced characterization of the material sample under examination by providing both differential amplitude and differential phase imaging of the sample. No known art exists for differential acoustic property measurements using a simple acoustic lens. The invention allows very sensitive measurement of local variations in acoustic properties and enhanced detectability of physical defects in the sample.
It is therefore a principal object of the invention to provide an improved non-destructive acoustic materials evaluation system.
It is another object of the invention to provide an improved SAM system and method.
It is a further object of the invention to provide an improved acoustic lens for use in a SAM.
It is yet another object of the invention to provide a differential property sensitive acoustic lens for non-destructive materials evaluation using a SAM.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, a differential property sensitive acoustic lens for non-destructive materials evaluation is described which in a preferred embodiment comprises first and second substantially semicylindrical shaped portions of fused silica disposed in closely spaced relationship along an axial plane, a substantially spherical depression defined in one end of each semicylindrical portion and a flat defined on each semicylindrical portion at the other end, a piezoelectric transducer attached to the flat of each semicylindrical portion, and a paraffin coated aluminum film of preselected thickness disposed between and in laminar contact with the semicylindrical portions for preventing acoustic and electrical cross talk between the transducers and between the semicylindrical portions.